1. Field of the Invention
The present invention relates to a magnetoresistive element and to a thin-film magnetic head, a head gimbal assembly, a head arm assembly, a magnetic disk drive, a magnetic sensor and a magnetic memory each of which incorporates the magnetoresistive element.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of magnetic disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a structure in which a write (recording) head having an induction-type electromagnetic transducer for writing and a read (reproducing) head having a magnetoresistive (MR) element for reading are stacked on a substrate.
MR elements include giant magnetoresistive (GMR) elements utilizing a giant magnetoresistive effect, and tunneling magnetoresistive (TMR) elements utilizing a tunnel magnetoresistive effect.
It is required that the characteristics of a read head include high sensitivity and high output capability. GMR heads incorporating spin-valve GMR elements have been mass-produced as read heads that satisfy such requirements. Recently, developments in read heads using TMR elements have been sought to conform to further improvements in areal recording density.
Typically, a spin-valve GMR element incorporates: a nonmagnetic conductive layer having two surfaces facing toward opposite directions; a free layer disposed adjacent to one of the surfaces of the nonmagnetic conductive layer; a pinned layer disposed adjacent to the other of the surfaces of the nonmagnetic conductive layer; and an antiferromagnetic layer disposed adjacent to one of the surfaces of the pinned layer farther from the nonmagnetic conductive layer. The free layer is a ferromagnetic layer in which the direction of magnetization changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer in which the direction of magnetization is fixed. The antiferromagnetic layer is a layer that fixes the direction of magnetization in the pinned layer by means of exchange coupling with the pinned layer.
Conventional GMR heads have a structure in which a current used for detecting magnetic signals (that is hereinafter called a sense current) is fed in the direction parallel to the plane of each layer making up the GMR element. Such a structure is called a current-in-plane (CIP) structure. On the other hand, developments have been made for another type of GMR heads having a structure in which the sense current is fed in a direction intersecting the plane of each layer making up the GMR element, such as the direction perpendicular to the plane of each layer making up the GMR element. Such a structure is called a current-perpendicular-to-plane (CPP) structure. A GMR element used for read heads having the CPP structure is hereinafter called a CPP-GMR element. A GMR element used for read heads having the CIP structure is hereinafter called a CIP-GMR element.
A read head incorporating the TMR element mentioned previously has the CPP structure, too. Typically, the TMR element incorporates: a tunnel barrier layer having two surfaces facing toward opposite directions; a free layer disposed adjacent to one of the surfaces of the tunnel barrier layer; a pinned layer disposed adjacent to the other of the surfaces of the tunnel barrier layer; and an antiferromagnetic layer disposed adjacent to one of the surfaces of the pinned layer farther from the tunnel barrier layer. The tunnel barrier layer is a nonmagnetic insulating layer that allows electrons to pass therethrough while the electrons maintain spins by means of the tunnel effect. The free layer, the pinned layer and the antiferromagnetic layer are the same as those of the spin-valve GMR element.
It has been reported recently that a nanocontact that connects two ferromagnetic materials to each other exhibits a great magnetoresistive effect. It is assumed that this magnetoresistive effect is produced by a quantum effect. An MR element utilizing the magnetoresistive effect will be hereinafter called a quantum MR element.
JP 2005-109241A discloses an MR element having such a structure that two ferromagnetic layers and a nanojunction (a nanocontact) therebetween are located in one plane. In addition, as a method of forming the MR element, this publication discloses a process of forming a patterned resist film on a layer made of a ferromagnetic material, and patterning the layer made of the ferromagnetic material by ion milling.
As disclosed in JP 2005-026699A or JP 2004-103769A, for example, a CPP-GMR element including a current-confined layer has been recently proposed as an MR element capable of obtaining high output signals. Typically, a current-confined layer has a plurality of minute conductive portions that penetrate in the direction of thickness, and insulating portions that separate the conductive portions from one another. In the CPP-GMR element disclosed in JP 2005-026699A, the current-confined layer includes a mosaic structure of the conductive portions and the insulating portions. In the CPP-GMR element disclosed in JP 2004-103769A, the current-confined layer is formed of a granular film including a conductive magnetic material and a dielectric material. In this granular film, the conductive magnetic material corresponds to the conductive portions, and the dielectric material corresponds to the insulating portions.
It is assumed that, in a quantum MR element, it is required to reduce the width of a narrowest portion of a cross section of a nanocontact for achieving a great magnetoresistive effect. In the MR element having a structure as disclosed in JP 2005-109241A, the nanocontact is formed by photolithography, for example. In this case, the width of the narrowest portion of a cross section of the nanocontact is defined by photolithography. Consequently, the MR element having such a structure has a problem that it is difficult to reduce the width of the narrowest portion of the cross section of the nanocontact because of the limitation imposed by the accuracy of photolithography.
In the CPP-GMR element including the current-confined layer as disclosed in JP 2005-026699A or JP 2004-103769A, the magnetoresistive effect depends on the size and the number of the conductive portions of the current-confined layer. However, it is difficult to precisely control the size and the number of the conductive portions of the current-confined layer. The CPP-GMR element therefore has a problem that it is difficult to manufacture elements having a desired magnetoresistive effect with reliability.